The lithium-ion battery cell is the premiere high-energy rechargeable energy storage technology of the present day. Unfortunately, its high performance still falls short of energy density goals in applications ranging from telecommunications to biomedical. Although a number of factors within the cell contribute to this performance parameter, the most crucial ones relate to how much energy can be stored in the electrode materials of the cell.
During the course of development of rechargeable electrochemical cells, such as lithium (Li) and lithium-ion battery cells and the like, numerous materials capable of reversibly accommodating lithium ions have been investigated. Among these, occlusion and intercalation materials, such as carbonaceous compounds, layered transition metal oxide, and three dimensional pathway spinels, have proved to be particularly well-suited to such applications. However, even while performing reasonably well in recycling electrical storage systems of significant capacity, many of these materials exhibit detrimental properties, such as marginal environmental compatibility and safety, which detract from the ultimate acceptability of the rechargeable cells. In addition, some of the more promising materials are available only at costs that limit widespread use. However, of most importance is the fact that the present state of the art materials only have the capability to store relatively low capacity of charge per weight or volume of material (e.g. specific capacity, (mAh/g); gravimetric energy density (Wh/kg−1); volumetric energy density, (Wh/l−1)).
Materials of choice in the fabrication of rechargeable battery cells, particularly highly desirable and broadly implemented Li-ion cells, for some considerable time have centered upon graphitic negative electrode compositions, which provide respectable capacity levels in the range of 300 mAh/g. Unfortunately, complementary positive electrode materials in present cells use less effective layered intercalation compounds, such as LiCoO2, which generally provide capacities only in the range of 150 mAh/g.
Intercalation compounds are not highly effective because the intercalation process is not an ideal energy storage mechanism. This situation occurs because of the limited number of vacancies available for lithium. An alternative process, reversible conversion, allows for all of the oxidation states of a compound to be utilized. The reversible conversion reaction proceeds as follows:nLi++ne−+Men+XnLiX+Mewhere Me is a metal and X is O−2, S2−, N− or F−. This reaction can lead to much higher capacities than can an intercalation reaction and, therefore, to much higher energy densities.
Badway et al. (Journal of The Electrochemical Society, 150(9) A1209-A1218 (2003)), for example, has described electrode materials having high specific capacities via a reversible conversion reaction. They reported specific capacities for carbon metal fluoride nanocomposites, such as a carbon FeF3 nanocomposite, active for this reaction, having >90% recovery of its theoretical capacity (>600 mA/g) in the 4.5-1.5 V region. They attained this major improvement in specific capacity by reducing the particle size of FeF3 to the nanodimension level in combination with highly conductive carbon.
Reversible conversion reactions may also be active for other metal fluorides. Bismuth fluoride, for example, is known to have a thermodynamic condition favorable for a 3V electrode material in lithium batteries, a voltage particularly useful for the development of a wide range of products from biomedical to telecommunications. Furthermore, the theoretical specific capacity, gravimetric energy density and volumetric energy density of bismuth fluoride exceed those of LiCoO2. The theoretical gravimetric and volumetric densities for BiF3 are, for example, 905 Wh/kg−1, and 7170 Wh/l−1, respectively, for the equation:BiF33LiF+Biwhereas such energy densities for the reactionLiCoO2LixCoO2+Liare only 560 Wh/kg−1 and 2845 Wh/l−1.
However, to date, bismuth fluoride has not been utilized as a positive electrode material in Li-ion battery cells. Most transition metal fluorides are insulators and possess little or no electrochemical activity as macromaterials. The present invention solves this problem by reducing the particle size of bismuth fluoride composites to the nanodimensional level in combination with a conductive matrix.